Let me bring Wikipedia here:
Windows Mobile is best described as a subset of Windows CE-based platforms. Currently, Pocket PC (now called Windows Mobile Classic), SmartPhone (Windows Mobile Standard) and PocketPC Phone Edition (Windows Mobile Professional) are the three main platforms under the Windows Mobile umbrella. Each platform uses various components of Windows CE, as well as additional features and applications suitable for their respective devices.
So, as I understand it, Windows CE is more of a "real" Windows for mobile devices, and Windows Mobile is somewhat trimmed and aimed at those who have less processor energy and memory. I actually did not come across some Win CE devices, perhaps because Microsoft is trying to install Windows XP and Vista, but not too successful with that.
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